Abstract
Solid-state spins provide a versatile physical platform for quantum information science because they combine atom-like quantum systems with the integration possibilities of a crystalline host. Among them, the negatively charged nitrogen-vacancy (NV) center in diamond is especially useful: its electronic spin can be optically initialized and read out, coherently manipulated by microwave fields, and coupled to nearby nuclear spins that can serve either as qubits, memories, sensors, or many-body environmental degrees of freedom. This dissertation explores several ways of using this hybrid structure. The central theme is that the same NV-based system can be viewed at different levels: as a small controllable register, as a sensor of a surrounding nuclear-spin environment, and as a route to engineer or exploit dark spin degrees of freedom.
The first part of the thesis establishes the physical and experimental basis of the platform. Starting from the symmetry of the NV center, the electronic orbitals, energy levels, optical transitions, intersystem crossing, spin initialization, and fluorescence readout are reviewed. The ground-state spin Hamiltonian is then connected to practical control protocols including optically detected magnetic resonance, Rabi oscillations, relaxation and coherence measurements, dynamical decoupling, nuclear-spin control, and correlation spectroscopy. The experimental implementation is summarized in a common framework, including the confocal optical path, microwave and radio-frequency control, timing architecture, and the specific setups used in the later chapters.
The second part demonstrates a variational quantum eigensolver using a two-qubit register formed by the NV electron spin and its intrinsic nitrogen nuclear spin. The experiment is performed under ambient conditions near the excited-state level anticrossing, where optical illumination enables initialization and fluorescence-based readout of the coupled electron–nuclear system. A hardware-efficient ansatz, direct Pauli-term measurements, parameter-shift gradients, and a gradient-descent update loop are used to minimize the target Hamiltonian H = X1X2 + Z1 + Z2. The measured energy converges toward the ground-state value after 16 iterations, and the final state reaches a fidelity of 98.9% with the ideal eigenstate. The remaining deviation is analyzed in terms of photon shot noise, readout error, magnetic-field inhomogeneity, and dephasing modeled by Lindblad simulations.
The third part develops quantum reservoir computing with a solid-state open spin system. Instead of requiring gate-level control over every spin, the protocol uses the optically accessible NV electron spin as a bright node for initialization, input encoding, and readout, while the surrounding 13C nuclear-spin bath supplies dark reservoir degrees of freedom. Numerical benchmarks on short-term memory and nonlinear autoregressive moving-average tasks show that more expressive readout improves both memory capacity and nonlinear prediction accuracy. Experimentally, radio-frequency pulses encode inputs into the nuclear-spin bath, the NV center reads out the bath response through phase accumulation, and a trained linear output layer reconstructs a nonlinear product task. Comparison with cluster-correlation-expansion simulations supports the interpretation that the useful nonlinear features originate from many-body spin dynamics rather than from classical post-processing.
The final part addresses the problem of initializing the dark nuclear-spin environment. A programmable hyperpolarization protocol is developed to transfer polarization from the optically reset NV electron spin to selected 13C nuclear spins through timing-engineered microwave control. By tuning the control sequence, the protocol realizes positive, negative, selective, and antiparallel nuclear-spin polarization configurations. Correlation spectroscopy verifies the generated polarization, while free-induction decay, Hahn echo, and ODMR measurements show that the engineered bath states modify the central-spin coherence and generate Overhauser fields with controllable sign. Together, these results show that dark spins in a solid-state environment need not be treated only as decoherence sources. With suitable control and readout, they become useful quantum resources for simulation, sensing, information processing, and many-body state engineering.